First the inhibition of the acto-HMM ATPase by tropomyosin alone will be investigated at high ATP concentration to determine whether the inhibition can be reversed by increasing the HMM concentration -- a result which would suggest that cooperative interaction of actin, HMM and tropomyosin occurs even at high ATP concentration. Second the activating effect which the troponin-tropomyosin complex has on the acto-HMM ATPase in the presence of Ca will be investigated to determine exactly which troponin components are necessary for this effect. We will also investigate whether this activating effect depends cooperatively on the HMM concentration both at high and low ATP concentration. Third we will continue our earlier studies in collaboration with Dr. E. Korn's group on the interaction of tropomyosin and troponin with the actin and myosin presently being isolated from Acanthamoeba -- an investigation which might reveal both how the troponin and tropomyosin effects depend on specific configurations of the actin and myosin and might also reveal how cellular actin differs from the actin found in organized skeletal muscle myofibrils.